For Federal Reserve Chair Janet Yellen, the current too-low inflation rate is not only “transitory,” it’s also “idiosyncratic.”

The head of the central bank this month expanded the lexicon to describe diminished price pressures in a bid to show she and most of her colleagues remain confident the unhealthy lack of inflation is only temporary. Officials were so sure prices would eventually rise closer to their 2 percent goal that they raised the benchmark interest rate so they wouldn’t have to tighten too quickly once inflation did flare.

Following the mid-month announcement of the first benchmark rate hike since 2006, Yellen named medical-care expenses and “non-market price increases, which are a little bit hard to understand” as two culprits for the subdued price environment.

“There are various idiosyncratic factors that affect core inflation,” she said at the Dec. 16 press conference. “But I personally don’t think we’re in a world where inflation is being determined in a different way than it has historically.”

That means she remains convinced that by having kept borrowing costs low to spur growth and reduce unemployment, the economy will automatically start generating slightly bigger price increases. As for health care, Yellen might be onto something.

Medical expenses have certainly done their part to depress price growth over the past couple years. Those costs rose just 0.9 percent in November from a year earlier, compared with 1.3 percent for the personal consumption expenditures price index excluding food and fuel — the so-called core measure.

Largely to blame for the slower health-care inflation in 2015 is that two years of government increases in reimbursement rates to physicians for Medicaid services, which tend to leak into what the private market charges, expired at the start of the year, said Omair Sharif, rates sales strategist at SG Americas Securities LLC in New York.

Policy makers are “looking through that kind of stuff because they know that these are sort of one-off legislative changes,” Sharif said. “Inflation should move higher because these things will be less of a drag going forward.”

Economists at Goldman Sachs Group Inc. in New York also wrote last month that they expect medical-care costs to rise early in 2016, in part as the year-over-year comparisons become more favorable and also because of more wage growth in health- care jobs.

As for “non-market prices,” Yellen probably was pointing to a number of imputed costs within the PCE price index that are less directly connected to the sort of supply-and-demand dynamics that would normally cause economists to worry about sluggish inflation.

Financial Services

Prices paid for “financial services and insurance” have a 7 percent share in the PCE index, so they can easily hold back the headline figures. That category includes investment advice and more complicated imputed costs such as the difference between interest paid on a checking account and what would be earned if that money was invested in government securities.

Costs for these financial services climbed 0.8 percent in November, the biggest gain in eight months, after falling 0.5 percent in October, the most since 2009. Such volatility is easy for policy makers to dismiss.

Yellen therefore has reason to believe inflation can make a comeback even as the central back begins to gradually raise interest rates.